Chapter 29 



THE DETERMINATION OF SEX 



One of the important problems to geneticists is reproduction 

 and especially that type of reproduction designated as "sexual 

 reproduction." It is so not only because all genes present in 

 succeeding generations become present in each individual at the 

 time of reproduction but also because the segregation of alleles 

 and the reassortment of genie combinations that make up so 

 much of the body of genetics occur only as a product of sexual 

 reproduction. In addition, of course, the genetic ratios obtained 

 from certain characters indicate that they are intimately con- 

 nected with sex. While geneticists have been studying the prob- 

 lem of sex ever since the science of genetics was founded, a defini- 

 tion of sex or of sexual reproduction is not easily constructed. 

 Sexual reproduction certainly involves certain cells called gam- 

 etes. They arise in animals directly by meiosis and in plants 

 from the gametophyte generation which came directly from 

 spores produced by meiosis. In either form, the gametes 

 have half or approximately half the number of chromosomes 

 found at some other stage in the life cycle. These gametes 

 usually unite by a process called fertilization, but sometimes 

 they produce a new individual without any such union. In some 

 of the lower forms of life the gametes that unite are alike mor- 

 phologically and in some organisms probably physiologically as 

 well, but in the higher animals and plants the gametes are dis- 

 tinctly different. Often the individuals in which one type of 

 gamete is produced differ in many morphological and physio- 

 logical respects from those in which the other type is found, but 

 in many plants even this distinction is largely obscured because 

 the two types of gamete-bearing plants are located in one sporo- 

 phyte. All these various illustrations can be classified as sexual 

 reproduction, which might be defined for our purposes as repro- 

 duction involving gametes. 



480 



